Bioinspired mechanical mineralization of organogels

Mineralization is a long-lasting method commonly used by biological materials to selectively strengthen in response to site specific mechanical stress. Achieving a similar form of toughening in synthetic polymer composites remains challenging. In previous work, we developed methods to promote chemical reactions via the piezoelectrochemical effect with mechanical responses of inorganic, ZnO nanoparticles. Herein, we report a distinct example of a mechanically-mediated reaction in which the spherical ZnO nanoparticles react themselves leading to the formation of microrods composed of a Zn/S mineral inside an organogel. The microrods can be used to selectively create mineral deposits within the material resulting in the strengthening of the overall resulting composite.


Chemical characterization
Nuclear magnetic resonance (NMR) spectra ( 1 H and 13 C) were recorded either on a Bruker Avance II+ 500 MHz spectrometer equipped with a Bruker QNP probe ( 1 H: 500 MHz, 13 C: 125 MHz) or a Bruker Ascend 9.4 T 400 MHz spectrometer equipped with an Avance III HD console and a BBFO "smart" probe ( 1 H: 400 MHz, 13 C: 100 Hz) at 25 °C.Chemical shifts (δ) are reported in parts per million (ppm). 1 H NMR spectra are referenced to TMS, while 13 C NMR spectra are referenced to either TMS or the residual solvent.
High resolution mass spectroscopy (HRMS) was conducted in an Agilent 6224 TOF LC/MS instrument using electrospray ionization (ESI) and a time of flight (TOF) detector.
Fourier transformed infrared (FTIR) spectra were recorded on a Perkin Elmer Spectrum Two FT-IR spectrometer in the MIR range (4000-400 cm -1 ) using the ATR mode.
X-ray spectra of the microrods were obtained in a KRATOS AXIS NOVA X-ray Photoelectron Spectrometer (XPS) and a Bruker D8 Powder X-ray Diffractometer (XRD).

Imaging techniques
Scanning electron microscopy (SEM) was conducted in a Carl Zeiss Merlin High-Resolution Field Emission Scanning Electron Microscope (FE-SEM).Samples were previously coated with a Pt film (10 nm thickness) using a Cressington 208 HR sputter coater with Pd/Pt target.
Scanning transmission electron microscopy (STEM) images were acquired using JEOL ARM 200F equipped with a cold field emission source operated at 200 kV.Energy dispersive X-ray (EDS) mapping was acquired using an Oxford X-Max 100TLE windowless SDD detector equipped with JEOL ARM 200F.
Light microscopy images were obtained in a Leica DM2700 P Polarization Microscope under brightfield and polarization modes.A few drops of tetrahydrofuran (THF) were used to swell the polymer composite samples prior to imaging.
Transmission electron microscopy (TEM) was conducted in a ThermoScientific Titan Krios G3i instrument.Cross-sections of polymer composite samples were obtained with a Leica EM UC6 Ultramicrotome.

Thermal and mechanical characterization
Differential scanning calorimetry (DSC) was conducted with a DSC 2500 instrument (TA Instruments) using a modulated DSC program in the range -90 to 250 ºC.Rheological measurements were conducted with a rotational rheometer (TA Instruments, ARES G2) equipped with a peltier element and a parallel plate fixture (25 mm).A volume of 100 μL of sample was used for each measurement, with a measuring gap of 250 μm, and the temperature set at 25 ºC.
Dynamic mechanical analysis (DMA) was conducted with a TA Instruments RSA-G2 DMA using the compression mode parallel disc fixture (25 mm).

Representative synthesis of the microrods
In a cylindrical polypropylene vial, McMT (66 mg, 0.50 mmol) was dissolved in DMF (400 μL).ZnO nanoparticles (20 mg, 0.25 mmol) were added to the solution and homogenously dispersed via sonication for 20 s.The mixture was sonicated (40 kHz) in the dark for 4 h.The product was diluted with methanol (5 mL) and separated by centrifugation (3000 x g for 10 min).The precipitate was washed twice with methanol (10 mL) and again separated by centrifugation.The product was dried at 50 ºC under vacuum overnight.Yield: 71 mg, 86 %.

Synthesis of Zn(McMT) n complex 1
A solution of Zn(NO 3 ) 2 ·6H 2 O (149 mg, 0.50 mmol) in water (1 mL) was prepared.In a separate vial, McMT (132 mg, 1.00 mmol) was dissolved in a NaOH (aq) 1 M solution (1 mL).The zinc nitrate solution was added dropwise to the McMT solution under vigorous stirring.A white precipitate formed, and the mixture was stirred for 30 min.The solid was recovered by filtration and washed thoroughly with water.The product was dried at 50 ºC under vacuum overnight.Yield: 128 mg, 78 %.

Control reactions
w/ respect to representative procedure for the synthesis of microrods ▪ No ZnO: sample without ZnO nanoparticles.▪ No US: sample was stirred in a vortex at 100 rpm.▪ w/ ZnBr 2 : instead of ZnO nanoparticles, anhydrous ZnBr 2 (56 mg, 0.25 mmol) was added and dissolved in the reaction mixture.▪ w/ McMT disulfide dimer: instead of McMT, the disulfide (66 mg, 0.13 mmol) was added and dissolved in the reaction mixture.

Synthesis of microrods within polymer organic solution
In a cylindrical polypropylene vial, poly(propylene glycol) tolylene
The reaction mixture was extracted with ethyl acetate (3 × 150 mL), and the combined organic layer was washed with water (3 × 100 mL) and brine (3 × 100 mL), dried over anhydrous Na 2 SO 4 and concentrated in vacuo to yield a light-yellow viscous oil.The product was further purified by flash column chromatography using an ISCO Teledyne instrument with a 330 g RediSep Rf silica column while eluting with a mixture of hexanes and ethyl acetate (gradient of 0 to 40 % ethyl acetate).Fractions were combined and concentrated in vacuo, yielding a pale viscous oil of bisphenol A di(3-azido-2 hydroxy propan-1-ol) ether (15.8 g, 37.1 mmol, yield 75 %).